1. Field of the Invention
The present invention relates to improvements in devices and methods for detecting the width of a printing medium in an image forming apparatus, and more particularly, to a printing medium width detecting device for a manual feeding unit, which has a plurality of sensors for automatically detecting the real width of the printing medium and avoiding unnecessary exposure, developing and transfer outside a region where printing is required, thereby reducing toner consumption and contamination of an image transfer unit.
2. Description of the Background Art
After a conventional image forming apparatus is switched on, a controller managing overall operation stores preset basic values regarding a size of a printing medium to be printed and conditions for the developing processing units prior to entering a ready state awaiting a print command. Typically, a standard size printing medium (e.g. paper) such as A4, Legal, or Letter selected by a user may be supplied to the image forming apparatus by an automatic printing medium feeding cassette. When a print command is generated, the controller receives information regarding the size of the printing medium in accordance with size detection switches provided on the side of the cassette mounting portion. The controller compares the received information with the preset basic values, and may store new values corresponding to the assumed size of the printing medium held by the inserted cassette in place of the preset basic values. Image data for one page is then generated by the controller in accordance with the recognized size of the selected printing medium, to produce an electrostatic latent image and transfer developing material onto the surface of a photosensitive dram in a region contacted by the printing medium.
A manual feeding unit is commonly employed when a user selects a non-standard size printing medium or frequently changes the size of the printing medium selected, such as that disclosed in U.S. Pat. No. 5,222,722 issued to Kamano on Jun. 29, 1993 and incorporated herein by reference. The controller for the imaging apparatus is generally unable to determine the width of a printing medium supplied by a manual feeding unit however, and image forming operations are performed based on preset basic or input values without regard to the actual width of the printing medium. No problems occur when the preset basic or input value is narrower than the actual width of the printing medium fed through the manual feeding unit. On the other hand, if the actual width of the printing medium is narrower than a width of image data exposed onto the photosensitive dram, developing material adheres to a region of the dram not contacted by the printing medium and is instead transferred to the surface of the transfer unit. This results in unnecessary toner consumption and contamination of the transfer unit, causing progressively deteriorating print quality.
A Method and Device for Saving Toner and Preventing Contamination in an Image Forming Apparatus is disclosed in U.S. patent application Ser. No. 08/535,874, claiming priority under 35 U.S.C. .sctn.119 on Korean Patent Application No. 25701/1994 and assigned to the assignee of the present invention, Samsung Electronics Co., Ltd. A controller for an image forming apparatus utilizes a sensed width of a manually fed printing medium to generate a latent image only in a reduced printing area. A slide bar attached to a variable resistor must be physically moved by a user to coincide with the edge of a manually fed printing medium to detect the width however, requiring extra effort by the user and potential mistaken adjustments.
U.S. Pat. No. 4,672,465 discloses an Image Forming Apparatus for detecting the shape of a paper sheet supplied to a manual feeding path of a thermal printer. A shape detecting apparatus is provided, having an array of light detecting elements and light emitting elements disposed respectively above and below a manual feeding path in a line parallel to the widthwise direction, so that the light transmission path for each emitter/detector pair is selectively blocked according to the width of the paper. A transparent printing medium such as MYLAR however, can not be detected by the disclosed light emitting elements. A mechanical alternative using an array of rotatable lever elements to actuate a lead switch is only briefly mentioned and would appear to be unreliable.